Enzymatic iodination was compared with chemical iodination of various copolymers and oligopeptides of Tyr. Iodination catalyzed by lactoperoxidase (LPO) or thyroid perioxidase (TPO) was always more efficient than iodination catalyzed by horseradish perioxidase or nonenzymatic iodination. A linear random copolymer of Lys and Tyr gave, with TPO or LPO, thyroxine (T4) in excellent yield. The kinetics of T4 formation with TPO was studied.